Applying accommodation versus sediment supply ratio concepts to stratigraphic analysis and zonation of a fluvial reservoir - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 106

progression of events in a fluvial system, or the
evolution of landforms to a condition of relative
instability; Stouthamer & Berendsen, 2000, 2007;
Postma, 2014) that operate over small spatial and
temporal scales.
Sequence stratigraphic analysis is included in
the concept of autostratigraphy developed for
fluvio-deltaic systems. Autostratigraphy is defined
as the stratigraphic non-equilibrium response gen-
erated by large-scale autogenic processes (Muto
et al ., 2007). These authors suggest that sequence
stratigraphy represents a limiting case of the more
general model of autostratigraphy and will be
applicable only if the periodicity of the unsteady
allocyclic factor is much smaller than the steady
dynamic allocyclic forcing. Some abrupt breaks in
the stratigraphic record are not necessarily associ-
ated with changes in allogenic conditions but can
result from purely autogenic processes of the
system. Therefore, any fluvial sequence strati-
graphic model needs to incorporate the combined
effects of A/S changes and large-scale autogenic
processes as well as short-term autogenic pro-
cesses such as avulsion.

used commonly in fluvial sequence stratigraphic
models. Accommodation (A) has a unit of length per
time and Sediment Flux (S) reflects the total volume
of sediments supplied for a given time interval and
is specified as a unit of volume per time (Muto &
Steel, 2000). Consequently, both A and S include the
notion of volume and time in their definition. A/S is
used as a number and stratigraphic changes in A/S
are interpreted as an expression of a change in posi-
tion of fluvial base level (similar to, for example,
Martinsen et al ., 1999). Therefore, it is necessary to
analyse how temporal changes in accommodation
in fluvial environments can be determined. As noted
above, A and S are intimately related but not neces-
sarily in equilibrium.

A/S change and recognition criteria

Changing A/S conditions are expressed in fluvial
stratigraphic datasets in a number of ways and a set
of criteria has to be established to detect readjust-
ment of fluvial equilibrium profiles and channel
planform style in (possible) response to increase
or decrease in non-marine accommodation (Ouchi,
1985; Leeder & Stewart, 1997; Currie, 1997;
Gawthorpe & Leeder, 2000; Bridge, 2003; Adams &
Bhattacharya, 2005; Schumm, 2005). Facies inter-
pretation (both interfluves and channel sandstones)
linked to analysis of temporal and spatial changes
in three-dimensional facies architecture are a key
issue as the most important tool for non-marine
correlation is to detect organised changes (trends)
in preserved facies. Several related parameters can
be used to detect these changes. However, care
must be taken because many of these parameters
are controlled by upstream as well as downstream
factors of source area, climate, differential subsid-
ence and eustacy (Schumm, 2005). It is therefore
important that they are not used in isolation. The
parameters include (see also Table 1):

Fluvial base level

The upper boundary of net accommodation
(lower limit to erosion or fluvial base level; for
definition see Appendix 1) is used as a measure
of stratigraphic change and as a stratigraphic
correlation level (base level). Its recognition is
complicated by the fact that the graded stream
profile is never in equilibrium at any spatial and
temporal scale (Schumm, 1977; Bull, 1991;
Holbrook et al ., 2006; Postma, 2014) and conse-
quently local autogenic factors have the potential
to overprint the stratigraphic signature (called
'autogenic shadowing' by Kjemperud, 2008). It is
therefore unlikely that the upper boundary of net
accommodation will have a tangible expression
in the fluvial rock record. Additionally, through
time several upper boundaries of net accommo-
dation will be preserved at different hierarchical
scales and therefore stratigraphic intervals have
to be identified that express similar stratigraphic
patterns of aggradation and/or degradation using
the concept of A/S.
The A/S concept is a useful way of analysing spa-
tial and temporal variations in depositional systems
(Jervey, 1988; Thorne & Swift, 1991; Schlager, 1993;
Shanley & McCabe, 1994; Allen et al ., 1996;
Martinsen et al ., 1999; Muto & Steel, 2002) and it is

1 Changes in the degree of palaeosol maturation.
These are related to variations in climatic fac-
tors such as total precipitation, seasonality,
temperature and the activities of organisms.
Cumulative palaeosols with only modest
degrees of palaeosol development are typical
of alluvial basins with rapid continuous
sedimentation.
2 Variations in the thickness of preserved palaeo-
sol profiles. These are also related to variations
in sedimentation rate and/or changes in accom-
modation generation.